The present invention relates to a control device for a proportionally adjustable hydraulic pump of a closed hydraulic circuit and to a variable displacement pump for a hydrostatic drive, the variable displacement pump being actuable hydraulically out of a zero position in two pivoting directions by means of a valve configuration, and the pump being, in particular, an axial piston pump. A valve configuration is provided for the adjustment of the hydraulic pump, serving for the activation of the adjusting piston from both sides.
Conventional electrohydraulic proportional adjustment systems for hydrostatic drives are known in many forms. To control the angle of adjustment of the piston of the variable displacement pump, it is necessary to provide feedback of an adjusting signal to the valve configuration. For this purpose, U.S. Pat. No. 5,205,201, for example, discloses an electrohydraulic proportional adjustment system with a mechanical feedback device between the servo piston and the control valve.
The principle underlying the circuit diagram of an electrohydraulic proportional adjustment system in accordance with the prior art, as disclosed by U.S. Pat. No. 5,205,201, is illustrated in FIG. 1 of the attached drawings. The axial piston pump 1 has a piston 3 that can be adjusted in two pivoting directions and is activated by means of a control spool 2. The control spool 2, for its part, is actuated by means of an electrically adjustable pilot valve 13. This known electrohydraulic proportional adjustment system furthermore has a mechanical feedback device 4, which feeds back the angle of adjustment of the piston 3 to the control spool 2 by means of a control spring 14 and feedback levers 15. This mechanical feedback of the pivoting angle to the control spool is complex in terms of design and furthermore requires a way of adjusting the zero position, this generally being made possible by means of mechanical levers. Not only is the construction of this control device complex but it is also necessary to carry out adjustment of the control spring and of the zero position of the piston, in each case by hand, after assembly in order to make precise control of the piston of the axial piston pump possible. The mechanical feedback levers furthermore result in tolerances in the precision of control, and these can lead to imprecise activation of the axial piston pump.
U.S. Pat. No. 5,881,629 has disclosed another control device for electrohydraulic proportional adjustment of axial piston pumps, in which the angular pivoting position of the piston of the axial piston pump is determined by means of angle sensors, the mechanical feedback device of the device described above thus being completely replaced by sensors. The position signal representing the pivoting angle is supplied by the angle sensors to an electronic control device, in particular a digital computer, by means of which the electrohydraulic valve is controlled, said valve in turn actuating the piston of the axial piston pump. The disadvantage is in each case that such angle sensors are sensitive to the effects of temperature and vibration, with the result that the reliability and precision of such a control device may be greatly impaired in certain areas of application.
DE-A1 43 37 667 also describes a control device of this kind with angle sensors for detecting the pivoting angle of an adjustable hydraulic machine. The same disadvantages occur as in the prior art already described. Moreover, a control device such as a microcomputer or a digital computer is required if angle sensors are used to detect the angular position of the piston of the axial piston pump.
It is therefore an object of the present invention to provide a control device and a variable displacement pump for a hydrostatic drive with electrohydraulic proportional adjustment that is as simple as possible in terms of its design and allows precise regulation or control of the pivoting angle of the piston of an axial piston pump. It is also an object of the present invention to provide a control system for variable displacement pumps that is simple and as economical as possible.